The present invention relates to ferromagnetic thin-film structures exhibiting relatively large magnetoresistive characteristics and, more particularly, to such structures used to couple digital signals from a source to an isolated receiver magnetically.
Many kinds of electronic systems make use of magnetic devices including both digital systems, such as memories, and analog systems such as field sensors. Magnetometers and other magnetic sensing devices are used extensively in many kinds of systems including magnetic disk memories and magnetic tape storage systems of various kinds. Such devices provide output signals representing the magnetic field sensed thereby in a variety of situations.
One use for such magnetic field sensors is the sensing of magnetic fields generated by electrical currents in a conductor as a basis for inferring the nature of such current giving rise to these fields. While this has long been done for magnetic fields generated by substantial currents, such sensing becomes more difficult to accomplish in lesser ranges of currents that include relatively small currents. The need for sensing fields due to such small currents arises, for instance, in situations where the currents generating the fields to be measured are provided merely as a basis for conveying signal information rather than for transmitting substantial electrical energy.
Such a situation occurs in many medical systems, instrumentation systems and control systems where there is often a need to communicate signals to system portions over signal interconnections from an external source or from another portion of the system. Often, the conductors carrying signal currents for such purposes must be electrically isolated from the portion of the system containing the sensor arrangement for those signals to measure the resulting magnetic fields. As an example, a long current loop carrying signal information in the loop current may, through lightning or static electricity discharges, become subject to having large voltage potentials relative to ground developed thereon. Such potentials must in many instances be kept from the signal sensing and receiving circuitry to avoid damage thereto even though that circuitry must still be able to capture the signal information contained in the loop current.
Signal isolators for these purposes are often preferably formed in monolithic integrated circuit chips for reasons of cost, convenience and system performance. In such an arrangement, one or more solid state magnetic field sensors are used to detect the magnetic fields provided by the currents containing the signals. A kind of magnetic field sensor which has been used in this situation is a Hall effect device. Such devices are often not satisfactory for sensing the magnetic fields due to small currents because of the limited sensitivity they exhibit with respect to magnetic fields.
Furthermore, there is often a lack of satisfactory remedial or supplementary measures in such arrangements for improving the limited sensitivity of Hall effect devices. The use of field concentrators is difficult to provide in a monolithic integrated circuit containing a Hall device because of the magnetically sensitive axis of that device being perpendicular to the directions the Hall device in the monolithic integrated circuit extends over the substrate supporting that device, i.e. the device axis of sensitivity is parallel to the thickness of the device rather than to the width or length thereof. Also information provided by Hall devices as to the magnetic fields measured thereby is in the form of a voltage which limits the use of such devices in bridge circuits which might otherwise be used for purposes of increasing the output signal providing the current signal information.
Another possibility in either hybrid integrated circuits or monolithic integrated circuits for signal isolation is the use of a light source having its electromagnetic radiation intensities controlled by signal currents from a signal source. Such a light source is electrically isolated from a light detector provided in the integrated circuit that is used to infer the nature of the signal currents from the light transmitted to and received thereby. Difficult engineering and economic problems make this an unsatisfactory solution as are various alternative capacitance based coupling solutions because of the same kinds of problems.
A further possibility has emerged in these circumstances for signal isolation in both hybrid integrated circuits and monolithic integrated circuits involving a current determiner comprising an input conductor, typically in some coiled configuration, and a current sensor both supported on a substrate adjacent to and spaced apart from one another so they are electrically isolated but with the current sensor positioned in those magnetic fields arising from any input currents in the input conductor. Such an isolated signals current determiner is an attractive device for these purposes in being both rapid in operation and economical in cost, and has been disclosed in U.S. Pat. No. 5,831,426 to W. C. Black and T. M. Hermann entitled Magnetic Current Sensor which is assigned to the same assignee and hereby incorporated herein by reference.
In the recent past, providing such current sensors as magnetoresistive effect based sensors in the form of an intermediate thin layer of an electrically conductive, nonmagnetic separating material having two major surfaces on each of which an anisotropic ferromagnetic thin-film is positioned has been found to lead to a "giant magnetoresistive effect" in the sensor if the thicknesses of the ferromagnetic thin-films and the intermediate layers in such a "sandwich" structure have been made sufficiently small. This effect can be enhanced by forming such sensors with additional alternating ones of these ferromagnetic films and intermediate layers to form superlattices. The resulting enhanced "giant magnetoresistive effect" can yield a magnetoresistive response which can be in the range of up to an order of magnitude greater than that due to the well known anisotropic magnetoresistive response. Sensors similar to those described herein for sensing magnetic fields external to those monolithic integrated structure devices containing such sensors are described in U.S. Pat. No. 5,617,071 to J. M. Daughton entitled "Magnetoresistive Structure Comprising Ferromagnetic Thin Films Having Magnetic Concentrator and Shielding Permeable Masses" and in an earlier filed co-pending application by J. M. Daughton entitled "Magnetic Structure with Stratified Layers" having Ser. No. 08/096,765, now abandoned, both assigned to the same assignee as the present application and hereby incorporated herein by reference.
Such an isolated signal current determiner can be used to couple digital data input signals provided in an input conductor to a receiver isolated from the input conductor, the input signals then being substantially replicated in a receiver circuit to provide replicas of those input signals at the receiver output. This is often a satisfactory arrangement for coupling digital data input signals into a system isolated from the source of the input signals, but in some situations too much power is consumed in providing those input signals. In other situations, a higher withstanding voltage is required in providing the signal isolation than has been available heretofore. Thus, there is a need for a signal isolation device exhibiting relatively high sensitivity, relatively high power efficiency and relatively high withstanding voltage, or some combination thereof, and which can be fabricated at a reasonably economic cost.